Material node - increase saturation one color?
you can separate by adding a math node, setting it to greater, and and add the solid color you want in the bottom input. Put your image texture in the top input and this will separate your image into black and white anywhere were there was blue (or any color you choose) will be black and the rest will be white. you can take the output of this node and put it into the saturation of a Hue/Saturation node. If you want to control the amount of saturation just put a math node between the Greater Than node and the Hue/Saturation node. I hope this helped you. :)